This invention relates to a processing method of making injection molded ferrous component parts having high strength by a two stage sintering process. The invented processing method is particularly suitable for making small component parts for miniaturized electrical and electronic equipment.
Injection molding of ferrous powders is a known art. For example, a carbonyl iron powder of less than 15 .mu.m average particle diameter is mixed with 8-15 weight % of a thermoplastic resin binder. This mixture is kneaded in a kneader at 135.degree. C. for three hours, and the mixture is then cooled to room temperature, followed by further granulation in the kneader to obtain feed particles of about 1-5 mm diameter for injection molding.
These particles at temperatures of 130.degree.-140.degree. C. are injection molded at an injection pressure of 500-700 kgf/cm.sup.2 at injection cycle times of 20-40 seconds.
The injected parts are then dewaxed at 300.degree.-500.degree. C., in a reducing atmosphere, such as in hydrogen, and this processing is terminated while the parts still retain about 10-20 weight % of the binder so that the dewaxed parts can retain the shape properly.
Dewaxing step is followed by further heating at 1100.degree.-1400.degree. C. to remove the remaining binder and to sinter the material in a hydrogen atmosphere, for example, to obtain the final sintered density of 50-70 % of the theoretical density. It is known that this high temperature dewaxing operation is time consuming (10-20 hours) and that the parts made by such a process are used as structural components in various electrical and electronic equipment.
The recent trends toward energy efficiency and light weight has promoted miniaturization of components and equipment in industries, and the demands for small thin walled structural components have increased. But the small components made by the conventional methods did not have sufficient strength to meet such requirements of the products.
The present invention resulted from extensive studies by the inventors to solve such problems of low strength and other problems associated with the conventional processing methods.
The inventors confirmed that the conventional dewaxing, in which the parts are heated at 300.degree.-500.degree. C., and the heating is terminated while fairly high amount of binder is still left in the molded parts (so as to maintain the shape), produces low density parts. They found that high density molded parts are obtained when dewaxing is carried out at a temperature range of 680.degree.-750.degree. C., higher than in the conventional method, to remove almost all the binder. At this stage, the carbon content of the dewaxed part becomes 0.05 weight %, and the theoretical density of the parts is about 60-70 %. The parts are then sintered at relatively low temperatures in the range of 870.degree.-910.degree. C., compared with the conventional sintering temperature of 1100.degree.-1400.degree. C. By this processing method, it is now possible to obtain parts having density values in the range of 96-99 % theoretical density in about 1-5 hours of sintering, in contrast to the conventional processing method involving long hours of heating to attain 90-95 % theoretical density.